This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2002-035604, filed Feb. 13, 2002, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to the material and structure of a cathode of an organic electroluminescence device having an electron injection layer between a light emitting layer and the cathode. More specifically, the present invention relates to an electrode structure for improving the efficiency of injection of electrons into a thin organic film, and to an organic electroluminescence device electrode structure having a two-layered structure including an ultra thin film of an insulating compound containing a lanthanoid, and a metal.
2. Description of the Related Art
As a man-machine interface in an information society, an organic electroluminescence device using an organic compound as an electroluminescence layer is beginning to be put into practice. An electroluminescence device is a light emitting device using electroluminescence. Compared to a liquid crystal display which is a light receiving type display currently put into practice, this organic electroluminescence device using an organic compound requires no back light since the device itself emits light. Therefore, the organic electroluminescence device is expected to be used as a display of a portable electronic apparatus or as a thin flat display.
A presently developed organic electroluminescence device has a two-layered structure between a metal electrode functioning as a cathode and a transparent electrode functioning as an anode. This two-layered structure is made up of a thin organic light emission film and thin organic hole transport film which are made of organic materials and stacked. The thin organic hole transport film has a function of facilitating injection of holes from the anode, and a function of blocking electrons.
Another presently developed organic electroluminescence device has a three-layered structure between the metal electrode and transparent electrode. This three-layered structure is made up of a thin organic electron transport film, thin organic light emission film, and thin organic hole transport film. The thin organic electron transport film has a function of facilitating injection of electrons from the cathode.
For example, Tang and VanSlyke reported an organic electroluminescence device having high luminance, a low driving voltage, a small size, and high efficiency, in C. W. Tang and S. A. VanSlyke: Applied Physics Letter, 51(12), pp. 913-915 (1987). This report was epoch-making in that the efficiency of the conventional organic electroluminescence device was improved by an order of magnitude by forming a very thin film of an organic emission material, from which an amorphous film is readily obtainable, by vacuum deposition. This device realized an external quantum efficiency of 1%, a luminous efficiency of 1.5 lm/W, and a luminance of 1,000 cd/m2 with a driving voltage of 10V or less. Also, this device achieved a low voltage by using a magnesium-silver alloy having a relatively small work function as a cathode.
At the present time after the elapse of more than 10 years from this report, higher efficiency and longer life are achieved, and a matrix panel using an organic electroluminescence device is put on the market.
In addition, Tang et al. reported the improvement of the luminous efficiency by mixing of a slight amount of a dye, such as a coumarin dye or pyran derivative, having high fluorescence quantum efficiency (C. W. Tang, S. A. VanSlyke, and C. H. Chen: Journal of Applied Physics, 65(9), pp. 3,610-3,616 (1989).
After that, Nakada and Tohma reported an organic electroluminescence device having higher efficiency. That is, they reported that a luminous efficiency of 12 lm/W and a luminance exceeding 100,000 cd/m2 were achieved by mixing a quinacridone derivative having high fluorescence quantum efficiency in a light emitting layer, and using an aluminum alloy containing lithium having a small work function as a cathode (H. Nakada and T. Tohma: Inorganic and Organic Electroluminescence (EL96 Berlin), (Edited by R. H. Mauch and H. E. Gumlich) pp. 385-390 (1996)).
An organic electroluminescence device is a device which generates excitons as electron-hole pairs by injecting electrons and holes into an organic film, and emits light by recombination of these electron-hole pairs. Accordingly, the emission intensity is generally proportional to the injection amount of electrons and holes. To realize a high-efficiency organic electroluminescence device, therefore, a large current must be injected with a low voltage. For this purpose, it is very effective to use a low-work-function metal which readily emits electrons at low voltages, as a cathode material. However, a low-work-function metal such as an aluminum alloy containing lithium is generally unstable and easily oxidizes in air. Hence, when used as an electrode of an organic electroluminescence device, the metal deteriorates fast. Therefore, it is being desired to use a stabler cathode material and develop an electrode structure.
To solve this problem, Wakimoto et al. successfully fabricated a stabler organic electroluminescence device by forming, on an organic layer, a very thin film 1 nm thick or less made of an oxide of a low-work-function metal such as lithium or cesium as a cathode material, and forming an aluminum electrode on this very thin film (T. Wakimoto, Y. Fukuda, K. Nagayama, A. Yokoi, H. Nakada, and M. Tsuchida: IEEE Transaction on Electron Devices, 44(8), pp. 1,245/1,248 (1997)). They reported that this structure made it possible to fabricate a high-efficiency organic electroluminescence device having high reproducibility.
Also, Hung et al. reported a high-efficiency organic electroluminescence device in which a fluoride layer of a very thin lithium film 1 nm thick or less was formed as a cathode material between an organic layer and aluminum cathode layer (L. S. Hung, C. W. Tang, and M. G. Mason: Applied Physics Letter, 70(2), pp. 153-154 (1997)).
In these organic electroluminescence devices of the above reports, however, insulating layers such as an oxide layer and fluoride layer are very thin. Therefore, a slight change in the thickness of this insulating layer poses problems such as an increase in the driving voltage and an increase of a non-light-emitting portion. That is, a thickness change of the insulating layer dramatically changes the characteristics of the organic electroluminescence device. Therefore, in the fabrication of an organic electroluminescence device on a large-area substrate, the uniformity of the thickness of the insulating layer is very important. Accordingly, it is presumably necessary to solve problems in the fabrication process.
The inventors of this application developed an organic electroluminescence device for providing an electroluminescence light emitting device which solved the above problems, operated at a low voltage, had a light emission surface having uniform emission intensity under the same conditions, and was capable of high-quality display, and filed a patent in 2001 (Jpn. Pat. Appln. KOKAI Publication No. 2002-289360). This organic electroluminescence device according to the above application has a substrate which is transparent in the visible range, an electrode formed on the substrate and transparent in the visible range, an organic layer formed on the electrode and having a hole transport function, an organic layer formed on the organic hole transport layer and having an electron transport function and light emitting function, and an electrode containing erbium formed on the organic layer having the functions of electron transport and light emission.
The present invention continues to the invention disclosed in the patent application described above, and provides an organic electroluminescence device which similarly operates at a low voltage and is also usable as a flat display. That is, it is an object of the present invention to solve the drawbacks of the conventionally used cathode materials, and provide an organic electroluminescence electrode structure having a more uniform light emission surface with a lower voltage, and capable of displaying images with higher quality.
As described in an embodiment, the present invention relates to an organic electroluminescence device wherein an anode, a hole transport layer made of an organic compound, a light emitting layer made of an organic compound, and a cathode are stacked in the order named, and an electron injection layer containing a lanthanoid oxide is formed between the light emitting layer and cathode.
As described in an embodiment, the present invention relates to an organic electroluminescence device wherein an anode, a hole transport layer made of an organic compound, a light emitting layer made of an organic compound, and a cathode are stacked in the order named, and an electron injection layer containing a lanthanoid fluoride is formed between the light emitting layer and cathode.
As described in an embodiment, the present invention relates to an organic electroluminescence device wherein an anode, a hole transport layer made of an organic compound, a light emitting layer made of an organic compound, and a cathode are stacked in the order named, and an electron injection layer formed by mixing a lanthanoid oxide and organic compound is formed between the light emitting layer and cathode.
As described in an embodiment, the present invention relates to an organic electroluminescence device wherein an anode, a hole transport layer made of an organic compound, a light emitting layer made of an organic compound, and a cathode are stacked in the order named, and an electron injection layer formed by mixing a lanthanoid fluoride and organic compound is formed between the light emitting layer and cathode.